Regarding W-CDMA (Wideband Code Division Multiple Access), LTE (Long Term Evolution), and LTE-A (LTE Advance), which are mobile communication standards, introducing a micro Home (e)Node B for expansion of service areas, personal use, or the like has been considered. Regarding the Home (e)NB, a UE (User Equipment) allowed to access a base station can be limited. The base station subjected to the limitation is referred to as a CSG (Closed Subscriber Group). Currently, standardization thereof is in progress by 3GPP (3rd Generation Partnership Project). The requirement for the Home (e)NB is defined in section 8 of Non-Patent Document 1.
Hereinafter, the 3GPP standard regarding the CSG is explained.
An owner or operator of the CSG can freely set the allowance or forbiddance of access to the CSG for each mobile station device. Additionally, a period of time for which an access to the CSG is allowed (hereinafter, expressed as an accessible time) can be set to a mobile station device to be allowed access. The accessible time is managed on the network side. If the accessible time expires, the mobile station device is forbidden to camp on the CSG cell. According to the standard of 3GPP Rel-8, if communication is in process when the accessible time expires, the communication call is forced to be disconnected.
If information concerning the allowance or forbiddance of access to the CSG is changed, that information is reported to the mobile station device. Based on this information, an allowed CSG list stored by the mobile station device is updated. However, the detailed procedure of updating the allowed CSG list has not been defined by the 3GPP.
When the mobile station device performs a handover to a CSG cell, the allowance or forbiddance of access to the CSG is determined by the following three steps.
Step 1) CSG identification information (CSG identity), which is reported by system information of the CSG cell, is obtained.
Step 2) It is confirmed whether or not the received CSG identification information is included in the allowed CSG list stored by the mobile station device.
Step 3) If the received CSG identification information is included, it is determined that access to the CSG cell is allowed. If the received CSG identification information is not included, it is determined that access to the CSG cell is not allowed.
In the case of LTE, each service frequency has a priority, which is taken in consideration when a peripheral cell is monitored and when cell reselection is evaluated. The correlation between service frequencies and priorities is included in and reported by system information from a base station device. The details of parameters reported by the system information are defined in Non-Patent Document 3.
The conditions for initiating monitoring a peripheral cell are defined in section 5.2.4.2 of Non-Patent Document 2 as follows.
1) If a frequency has a priority higher than that of a frequency of a serving cell: monitoring is always performed.
2) If a frequency has a priority equal to or lower than that of the frequency of the serving cell: monitoring is performed only when the reception quality of the serving cell becomes lower than a given threshold.
Additionally, a cell reselection evaluating method is defined in sections 5.2.4.5 and 5.2.4.6 of Non-Patent Document 2 as follows.
a) If a priority of a frequency of a cell to be subjected to reselection evaluation>a priority of a frequency of a serving cell: a handover to an evaluating cell is performed if the reception level of the evaluating cell (SnonServingCell, x) is maintained higher than a threshold (Threshx, high) for a given period of time (TreselectionRAT).
b) If the priority of the frequency of the cell to be subjected to reselection evaluation=the priority of the frequency of the serving cell: cells are ranked according to the reception levels, and a handover to an evaluating cell is performed if a state, in which the rank of an evaluating cell exceeds the rank of the serving cell, is maintained for a given time (TreselectionRAT).
c) If the priority of the frequency of the cell to be subjected to reselection evaluation<the priority of the frequency of the serving cell: a handover to the evaluating cell is performed if a state, in which the reception level of the serving cell (SServingCell) is lower than a given threshold (Threshserving, low) and the reception level of the evaluating cell (SnonServingCell, x) is higher than another threshold (Threshx, low), is maintained for the given time (TreselectionRAT).
Regarding cell selection/reselection in the cases of the GSM, the W-CDMA, and the LTE, a concept for selecting a cell having the highest reception level, which is referred to as “best cell principle,” is used. Additionally, in the case of the LTE, a frequency having a high priority at the time of cell search is always to be monitored. Further, as the priority is higher, the handover condition is more relaxed, thereby making a frequency having a higher priority be more likely to be selected. Regarding allocation of cells to one frequency band, there are following three patterns.
Pattern A) Only a CSG is allocated in the frequency.
Pattern B) Only a normal cell (i.e., macrocell) is allocated in the frequency.
Pattern C) A macro cell and a CSG are allocated in the frequency.
In 3GPP RAN 2 (TSG RAN WG2; Technical Specification Group Radio Access Network Working Group 2), it is agreed that an implicit priority is used at the time of cell reselection (Non-Patent Document 4). The concept of the implicit priority includes the following two rules.
Rule 1) If the reception level of a CSG is the highest in a frequency, the priority of that frequency is regarded to have the highest value.
Rule 2) If a serving cell is a CSG, the priority of a frequency thereof is regarded to have the highest value.
Regarding the introduction of the implicit priority, it has not been defined how to select a cell if a CSG is a cell having the highest reception level among multiple frequencies. It is agreed in RAN 2 that the implementation of operations in such a case are dependent on each mobile station device.
Different from the LTE, Patent Document 1 discloses a method in which a priority is set to each cell, and a terminal determines a handover destination based on the priorities of cells. In the case of the LTE, however, a priority is not set to a cell. Additionally, even if the priorities of frequencies are regarded as the priorities of cells, multiple CSGs have the same priority under such a condition that the implicit priority is used. For this reason, the conventional method cannot be applied.